The present invention generally relates to communication systems, and in particular to a system and method for training a power amplifier residing in a linear transmitter that employs supply modulation for facilitating efficient operation of the linear transmitter.
Increasing demand for mobile and personal communications services has renewed interest in spectrally efficient modulation schemes. However, due to growing pressures for extra capacity, the advantages of retaining a constant envelope are giving way to linear modulation. Although filtering applied in linear modulation schemes generally produces gains in spectrum utilization, envelope variations are also introduced. Additionally, the inefficiency of conventional linear RF amplifiers has a deleterious effect on battery life of portable wireless transmitting equipment. Improvements in RF amplifier efficiency would directly lead to improvements in the talk-time recharging intervals and size and weight of the overall wireless unit. The ideal amplifier for linear modulated portable systems is therefore a linear amplifier which is also power efficient.
Linear transmitters are well known. To achieve both linearity and efficiency in such devices, linearization techniques can be employed in a power amplifier such as a Cartesian feedback loop. A Cartesian feedback loop is a closed loop negative feedback technique which sums the baseband feedback signal to quadrature component signals (e.g., in-phase (I) and quadrature (Q) signals) prior to amplifying and up-converting to an output frequency and a power level. Cartesian feedback of the baseband quadrature modulation provides excellent reduction in intermodulation distortion with low complexity and cost.
High level modulation of the power supply of an RF power amplifier, for the purpose of amplitude (or envelope) modulation is a known technique which provides good power efficiency when combined with efficient high level power supply modulators. A problem with these systems is the relatively larger bandwidths of the amplitude signals compared with corresponding baseband quadrature modulation and the inability to control AM-PM distortion. Consequently, the spectral control is generally inferior to that of Cartesian Feedback, IF Feedback and RF Feedback. Full Polar Feedback overcomes the distortion problem but still suffers from the bandwidth expansion problem. Additional problems occur when power supply modulation is run too close to the saturation point of the power amplifier because inaccuracies in component gain due to manufacturing tolerances and variations in temperature, power amplifier loading and supply voltage may allow one amplifier to operate to specification, while another amplifier may saturate under the same conditions.